The present invention relates generally to systems and methods for the handling of workpieces. More particularly, the invention relates to a system and method for accurately locating and positioning circular workpieces, such as semiconductor wafers.
The need to locate and position circular workpieces can arise under a variety of circumstances. For example, semiconductor wafer when the retrieved from wafer cassettes are frequently not centered and are randomly oriented with regard to their crystalline lattice structure. Usually, it is necessary to position individual wafers prior to processing, such as plasma etching, chemical vapor deposition, and the like.
Unfortunately, such positioning is problematic. Prior systems have often used either fixed or movable pins to engage the periphery of the wafer and provide some degree of centering. The use of pins, however, is inaccurate, particularly when the wafer being centered has a diameter deviating from the nominal diameter. Moreover, contact with the pins causes substantial generation of particulates which is undesirable in virtually all semiconductor processes. Other systems for locating and aligning semiconductor wafers employ multiple arrays of photosensitive detectors beneath the wafer to locate its periphery. Such systems, however, require a minimum of three detector arrays and are limited by the accuracy with which those arrays may be located. Additionally, these systems require a separate mechanism for correcting the positions of the wafers subsequent to locating them.
For the above reasons, it would be desirable to provide methods and systems which are capable of accurately detecting the positions of circular workpieces, such as semiconductor wafers, and for aligning such workpieces according to preselected criteria. The method and system should be highly accurate, avoid the generation of particulates, and be able to provide true centering regardless of any deviations between the actual and nominal diameters of the wafer. In addition, the system should be relatively simple, inexpensive to build and operate, and be compatible with other material handling and processing systems, such as semiconductor wafer processing systems.